North Korea will be able to field a reliable, nuclear-capable intercontinental ballistic missile as early as next year, U.S. officials have concluded in a confidential assessment that dramatically shrinks the timeline for when Pyongyang could strike North American cities with atomic weapons.

The new assessment by the ­Pentagon’s Defense Intelligence Agency (DIA), which shaves a full two years off the consensus forecast for North Korea’s ICBM program, was prompted by recent missile tests showing surprising technical advances by the country’s weapons scientists, at a pace beyond what many analysts believed was possible for the isolated communist regime.

The U.S. projection closely mirrors revised predictions by South Korean intelligence officials, who also have watched with growing alarm as North Korea has appeared to master key technologies needed to loft a warhead toward targets thousands of miles away.

The finding further increases the pressure on U.S. and Asian leaders to halt North Korea’s progress before Pyongyang can threaten the world with nuclear-tipped missiles. President Trump, during his visit to Poland this month, vowed to confront North Korea “very strongly” to stop its missile advances.

The DIA has concluded that North Korean leader Kim Jong Un will be able to produce a “reliable, nuclear-capable ICBM” program sometime in 2018, meaning that by next year the program will have advanced from prototype to assembly line, according to officials familiar with the document. Already, the aggressive testing regime put in place in recent months has allowed North Korea to validate its basic designs, putting it within a few months of starting industrial production, the officials said.

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The DIA and the Office of the Director of National Intelligence declined to address any classified assessments.

But Scott Bray, ODNI’s national intelligence manager for East Asia, said in a statement: “North Korea’s recent test of an intercontinental range ballistic missile — which was not a surprise to the intelligence community — is one of the milestones that we have expected would help refine our timeline and judgments on the threats that Kim Jong Un poses to the continental United States. This test, and its impact on our assessments, highlight the threat that North Korea’s nuclear and ballistic missile programs pose to the United States, to our allies in the region, and to the whole world. The intelligence community is closely monitoring the expanding threat from North Korea.”

One of the few remaining technical hurdles is the challenge of atmospheric “reentry” — the ability to design a missile that can pass through the upper atmosphere without damage to the warhead. Long regarded as a formidable technological barrier for impoverished North Korea, that milestone could be reached, beginning with new tests expected to take place within days, U.S. analysts said. U.S. officials have detected signs that North Korea is making final preparations for testing a new reentry vehicle, perhaps as early as Thursday, a North Korean national holiday marking the end of the Korean War.

“They’re on track to do that, essentially this week,” said a U.S. official familiar with the intelligence report who, like others, insisted on anonymity to discuss sensitive military assessments.

North Korea has not yet demonstrated an ability to build a miniaturized nuclear warhead that could be carried by one of its missiles. Officials there last year displayed a sphere-shaped device the regime described as a miniaturized warhead, but there has been no public confirmation that this milestone has been achieved. Preparations reportedly have been underway for several months for what would be the country’s sixth underground atomic test. The last one, in September, had an estimated yield of 20 to 30 kilotons, more than double the explosive force of any previous test.

North Korea startled the world with its successful July 4 test of a missile capable of striking parts of Alaska — the first such missile with proven intercontinental range. The launch of a two-stage “Hwasong-14” missile was the latest in a series of tests in recent months that have revealed startlingly rapid advances across a number of technical fields, from mastery of solid-fuel technology to the launch of the first submarine-based missile, current and former intelligence officials and weapons experts said.

(Anna Fifield,Jason Aldag/The Washington Post)

“There has been alarming progress,” said Joseph DeTrani, the former mission manager for North Korea for the Office of the Director of National Intelligence and a former special envoy for negotiations with Pyongyang. “In the last year they have gained capabilities that they didn’t have, including ones that we thought they would not have been able to obtain for years.”

The July 4 missile test also caught South Korea’s intelligence service off guard, prompting a hasty revision of forecasts, according to South Korean lawmakers who have received closed-door briefings. “The speed of North Korea’s ICBM missile development is faster than the South Korean Defense Ministry expected,” said lawmaker Lee Cheol-hee of the left-wing Minjoo party, who attended an intelligence committee briefing after the July 4 test.

The South Korean government, which is actively trying to engage the regime in Pyongyang, has declined to call the most recent test a success. North Korea still has not proved it has mastered some of the steps needed to build a reliable ICBM, most notably the reentry vehicle, Lee said.

Still, officials across the political spectrum acknowledged that North Korea is rapidly gaining ground. “Now they are approaching the final stage of being a nuclear power and the owner of an ICBM,” said Cha Du-hyeogn, who served as an adviser to conservative former president Lee Myung-bak.

U.S. spy agencies have detected multiple signals that North Korea is preparing to test a reentry vehicle. Analysts believe that the July 4 test was intended to demonstrate range — the ability of its new two-stage ICBM prototype to reach altitude and distance milestones — while the new launch will seek to validate engineering features designed to protect the warhead as it passes through the upper atmosphere and then is delivered to a distant target.

The latest designs appear to cobble together older systems — including portions of a missile frame used to launch satellites into orbit — with a more advanced engine that North Korea began testing earlier this year. Much of the technology is based on old Soviet-era designs that have been reworked by what U.S. experts describe as an increasingly capable cadre of homegrown engineers, goaded along by a leadership that has pursued nuclear weapons and delivery systems with single-minded zeal.

Kim vowed in January to successfully test a nuclear-capable ICBM in 2017, achieving a long-sought goal that North Koreans believe will serve as the ultimate deterrent against threats to the communist regime’s survival. At the time, the U.S. intelligence community’s formal assessment still held that a credible ICBM threat would not emerge until 2020 at the earliest.

“North Korea’s timeline moved faster than we expected,” said the U.S. official familiar with the new DIA assessment. “We weren’t expecting an ICBM test in July.”

Former U.S. officials and weapons experts said a successful test of a nuclear-capable ICBM would dramatically raise the stakes in the North Korean crisis, putting new pressure on North Korea’s neighbors and increasing the risk of miscalculation. “The danger is that decision time and warning is greatly reduced when North Korea has the weapons, and that escalation can happen quickly,” said Jon Wolfsthal, senior director for arms control and nonproliferation with the Obama administration’s National Security Council.

The specter of a nuclear-armed, ICBM-capable Kim “takes the risk to a new level but does not change the nature of the threat we have faced for some time,” Wolfsthal said. “We have to deter North Korea from ever using any nuclear weapons and make clear that any move to use these weapons is suicide.”